The present invention relates to a novel linear and cross-linked polybenzimidazole formed by the condensation of certain polyamines and anhydrides and to their method of preparation and use.
For several years, the thermal and oxidative stability of polymers containing aromatic structures have been recognized and a variety of polymers have been synthesized in which aromatic rings are linked together or combined with aliphatic segments in chains or networks. The synthesis of a variety of polybenzimidazoles has been previously reported. (Encyclopedia of Polymer Science and Technology, Vol. II, pages 188 to 228).
The more familiar polybenzimidazoles are obtained from bis aromatic ortho tetraamines and aromatic diacids or their derivatives by either a melt condensation or reaction in solvent at temperatures generally above 200.degree. C. The condensation of an unsubstituted aromatic ortho diamine with a cyclic anhydride of a diacid however does not provide the benzimidazole as a single product. The products obtained are shown below by example with model compounds. (F. Dawans and C. S. Marvel, "Polymers from ortho Aromatic Tetraamines and Aromatic Dianhydrides", Journal of Polymer Science: Part A, Vol. 3, pp. 3549-3571 (1965). ##STR3##
Accordingly, benzimidazole polymers of sufficient molecular weight as to be useful are difficult to obtain with these reactants. Even though the reaction of phthalic anhydride with 3,3',4,4'-tetraaminebiphenyl to provide a linear polybenzimidazole is reported in U.S. Pat. No. Re. 26,065 (3,174,974), it is evident from the publications described below that some portion of the phthalic anhydride was consumed as a chain terminating agent and was not used to propogate the molecular weight growth of the polymer.
In addition, a new polymer is obtained from dianhydrides and tetraamines if a precise heating schedule is employed. The above Dawans and Marvel work describes a high molecular weight polybenzoylenebenzimidazole from pyromellitic dianhydride and 3,3',4,4'-tetraaminobiphenyl. Careful slow heating of the reactants to temperatures of 300.degree. C. to 350.degree. C. is required. If heating is too rapid, a cross-linked polybenzimidazole structure is obtained. The two polymers which can be obtained require different stoichiometries of reactants to proceed to high molecular weight. This is illustrated below: ##STR4##
Colson, Michel, and Paufler further elucidated the intermediates formed in the polymerization of ortho diamines with dianhydrides. (J. G. Colson, R. H. Michel, and R. M. Paufler, "Polybenzoylenebenzimidazoles", Journal of Polymer Science: Part A-1, Vol. 4, 59-70 (1966).
U.S. Pat. No. 3,708,439 describes polybenzimidazoles with the following structure: ##STR5## where R.sub.1 is aromatic and R.sub.2 is described as being formed from diphenyl esters of diacids, wherein R.sub.2 is arylene (C.sub.6-12), alkylene (C.sub.1-12), cycloalkylene (C.sub.5-8), or ##STR6## In all instances, only the m or p substituted acids are described. Thus, the patent excludes reference to condensation with the inexpensive common diacid derivative, phthalic anhydride. This is possibly because of the foregoing disclosure in Dawans et al. that phthalic acid does not form useful polymers with certain diamines.